Polyamide resins have excellent mechanical strength, oil resistance, abrasion resistance, thermal resistance, etc. and have hitherto been utilized in great quantities, due to the polyamide resins being one of the most typical of the engineering type plastics. However, polyamides have disadvantages in that polyamides have certain properties which are poor as compared with other plastics, such as properties related to dimensional stability, moisture absorption, heat distortion resistance under high load, and impact resistance in a dry state.
In contrast thereto, polyphenylene ether resins are resins having excellent dimensional stability, electrical properties, heat distortion resistance under high load, water resistance, etc., and have been widely used industrially in the form of a blend with polystyrene resins. Polyphenylene ether resins, however, have serious disadvantages in that these resins have poor oil resistance and molding flow properties.
Thus, attempts have been made to blend these two resins for the purpose of combining the advantages respectively inherent in each of these resins and minimize or eliminate the disadvantages characterized in the resins individually. Heretofore, a great number of such resin combinations have been proposed. Such resin combinations include those obtained by simply blending the polyamide and polyphenylene ether resins, in particular, by melt blending these resins (as disclosed in U.S. Pat. No. 3,379,792 and U.S. Pat. No. 4,338,421). However, polyphenylene ethers and polyamides are hardly compatible when blended, and as a result, it is impossible to obtain from such a simple blend a molded product which would have excellent mechanical strength.
Also known is a composition obtained by compounding as a compatibility-improving agent, said agent comprises a copolymer of a styrene compound with an .alpha.,.beta.-unsaturated dicarboxylic acid anhydride, together with polyphenylene ether and polyamide, and further adding a rubber-like substance as an impact resistance-reinforcing agent (as disclosed in U.S. Pat. No. 4,339,376). Although in this composition the mechanical performance or oil resistance can be enhanced by virtue of improving the compatibility of the polyamide resin and the polyphenylene ether resin with each other, the attachment (presumably by graft polymerization) between these two resins through the compatibility-improving agent, would become excessive and render the dispersion between the two resins so extremely fine such a composition may be accompanied with disadvantages, such as the molding flow properties may be extremely lowered and the heat distortion resistance under high load may not be capable of being sufficiently enhanced.
There have been further proposed a process for producing a modified polymer which comprises adding to a polyamide and a polyphenylene ether, a compound having (a) a carbon-carbon double bond or a carbon-carbon triple bond and (b) a functional group such as a carboxyl group and an acid anhydride group in the molecule thereof, followed by melt kneading (as disclosed in U.S. Pat. No. 4,315,086), and a composition to which a rubber-like substance is further added (as disclosed in Japanese Unexamined Patent Publication No. 49753/1981). In these proposals, however, there are disadvantages in that sufficient impact resistance cannot be achieved and in that the molding flow properties obtained are inferior.
In other words, up to the present time, no conventional prior art process has been discovered for obtaining a resin composition which is excellent and well balanced in each of the impact resistance, heat distortion resistance, stiffness and molding flow properties.
Under such circumstances, it is an object of the present invention to provide a thermoplastic resin composition excellently balanced in heat distortion resistance, impact resistance, oil resistance, stiffness, molding flow properties, etc. in a blend system which comprises polyamide and polyphenylene ether.
The present inventors have made intensive studies to achieve the above purpose and as a result, found that the purpose can be achieved by controlling in a particular state, the dispersion form of the three components in a composition which comprises in a specified proportion a polyamide, a polyphenylene ether and, as a reinforcing agent, a rubber-like substance having a specified structure. The present invention has thus been accomplished based on such a finding.